1. Field of the Invention
This invention relates to compositions useful for insulating wire and cable. More specifically, the compositions are crosslinkable, flame retardant compositions of copolymers of ethylene with vinyl carboxylates, alkyl acrylates or alkyl methacrylates, hydrated inorganic fillers, coupling agents, stabilizers and a peroxide/scorch retarding additive combination consisting essentially of α,α′-bis(t-butylperoxy)diisopropyl benzene and 2,4-diphenyl-4-methyl-1-pentene. The flame retardant compositions make it possible to achieve faster cures while retaining acceptable scorch.
2. Description of the Prior Art
Crosslinkable polymers, such as ethylene-vinyl carboxylate and alkyl(meth)acrylate copolymers, formulated with hydrated inorganic fillers, one or more stabilizers and one or more crosslinking agents are widely used fire retarding insulations for wire and cable. Other additives such as pigments, processing oils, lubricants and coupling agents can also be included in these formulations. Compositions of this type which find use as single layer insulation and jacketing for copper wire are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,422 to North, et al., and U.S. Pat. Nos. 4,349,605 and 4,381,362 to Biggs, et al.
A wide variety of organic peroxides are known to be effective crosslinking agents; however, in recent years with emphasis on increasing coating line speeds the focus has been on the use of high activity peroxides which provide acceptable cure rates at high line speeds. There are, however, some problems associated with their use. One particularly troublesome feature is the tendency of high activity peroxides to scorch. Scorching is the undesirable premature crosslinking of the formulation during compounding and extrusion, i.e., mixing and coating of the wire prior to curing of the coated wire in the vulcanization step.
Premature crosslinking during processing results in the formation of undesirable gel particles in the melt. The presence of gel particles can result in rough surface appearance of the extruded insulation. In addition to detracting from the aesthetics of the coated wire, these regions of irregularity are often more susceptible to electrical failure or mechanical breakdown. Any premature crosslinking during processing also increases the amount of work necessary to process the formulation. This, of course, results in higher energy costs for the processor.
Possibly the biggest problem with the use of high activity peroxides is that they limit the flexibility of the processor during coating line interruptions. Insulation compositions formulated with these peroxides have a relatively short processing window. In a worst case scenario, if production is interrupted for a period beyond the useful life of a batch being processed, the entire batch must be removed from the mixer and scrapped.
One approach to expanding the processing window and extending scorch times has been the use of so-called scorch retardants. Monomeric vinyl compounds, such as α-methyl styrene and lauryl or stearyl methacrylate, are disclosed in U.S. Pat. No. 3,954,907 to be useful scorch retardants for ethylene polymer compositions. 2,4-Diphenyl-4-methyl-2-pentene, a dimer produced by the dimerization of α-methylstyrene, is also disclosed in U.S. Pat. No. 5,298,564. Neither reference, however, deals with flame retardant compositions and, more particularly, compositions which are rendered flame retardant as a result of the incorporation of high levels of hydrated inorganic fillers therein.
The ability to extend scorch time is particularly advantageous with insulation compositions which are flame retardant as a result of the inclusion of high levels of hydrated inorganic fillers since rigorous processing is required to incorporate and uniformly disperse the filler within the polymer matrix.
It would be highly desirable to have flame retardant insulation compositions having rapid cure rates and significantly improved scorch times. These objectives are achieved with the compositions of the present invention which will be described in more detail to follow.